Switching power converters are often used when an input voltage needs to be converted to another voltage level. When stepping down voltages, a buck converter is often used.
FIG. 1 illustrates a block diagram of a conventional buck converter 100. FIG. 2 illustrates timing waveforms for the buck converter of FIG. 1. An error amplifier 110 compares a feedback signal from the output Vout to a reference voltage. A comparator 130 compares an output of the error amplifier 110 to an output from a sawtooth waveform generator 120. An output of the comparator 130 is used by a switching controller 140, which adjusts a duty cycle of a pulse-width-modulation signal, which controls switch SW1. The switch SW1 drives a network including an inductor L1, a diode D1 and a capacitor C1.
As shown in FIG. 2, signal 220 shows the sawtooth signal that is compared to the output of the error amplifier 110, which is indicated by signal 210. As shown, the buck converter 100 is in a stable state with the error signal 210 mostly stable; indicating the output voltage Vout matches the reference voltage Vref. The pulse-width-modulation signal 235 goes low, turning off the switch SW1, when the sawtooth signal 220 exceeds the error signal 210 as indicated by line 240. The pulse-width-modulation signal 235 goes high, turning on the switch SW1, either from a clock signal (not shown) or when the sawtooth signal 220 is lower that the error signal as indicated by line 250.
The output (not shown) of the switch SW1 will oscillate based on the pulse-width-modulation signal 235, which is then filtered by the network to create a stable voltage on the Vout signal.
In switching power converters, for example buck or boost converters, a common means of shaping the feedback loop frequency response is to use current feedback to modify the response of the duty cycle modulator. This current feedback may take the form of a measure of the instantaneous power switch current in a form that can properly modify the stabilizing slope signal. Alternatives for developing this current feedback traditionally involve multiple replica currents, which can cause additional cost, inaccuracy, a reduction in speed, and increased complexity.
Some switching power converters may also compare the switch current to a reference level and use the result to shut off the main switch when a current overload causes the switch current to exceed a level determined by the reference current. Another problem may occur when the voltage across the inductor, during the switch off time, is insufficient to allow the inductor current to ramp down as much as it ramps up during the minimum on time of a current limited cycle. If this unbalanced ramping happens and normal switching continues, inductor current may be increased somewhat at the end of each cycle and may build up to values excessively beyond normal current limiting detection. In this case, a special detection and response, other than that of normal current overload, may be needed.
In conventional switching power converters, for example a buck or a boost converter, there is often a need to provide both frequency compensation, for normal closed loop stable operation, and also to provide a soft start function. Each of these functions conventionally has been implemented using a different capacitor for each function. Loop stabilizing frequency compensation is often conventionally shaped by loading an Operational Transconductance Amplifier (OTA) with a selected driving point impedance. Frequently, this compensation is done with a single series RC network as the driving point impedance. In addition, soft start functions are often included in a switching power converter. Many conventional implemenations include a switch to preset the voltage to result in the minimum duty ratio. The voltage is then made to rise slowly so as to cause the duty ratio to change slowly and gradually bring the converter ouput to the normal controlled level.
There is a need for methods and apparatuses for switching power conversion that provide a simplified process and circuit that combines features of frequency compensation and a soft start output process.